Structural Precast Elements:

Award:

PCI 2007 Design Award Co-Winner, Total-Precast Concrete Design

To design the new polymers production facility for Eastman Chemical Co., designers typically would have used steel components. “But as we developed this project,” says Pete Primm, senior vice president for the Chemicals Business at the architectural firm of CH2MHill Lockwood Greene, “it became evident that this was not your normal project.” By working closely with the precast manufacturer and erector, the designers produced a total-precast concrete structural system that met the unusual challenges and provided additional benefits.

Key considerations were loading requirements and budget for the specialized uses for which the facility would be used, he explains. These included the need for a 134-ft-tall structure with an average floor-to-floor height of 24 ft. The structure had to accommodate massive, unique, and sensitive process equipment with multiple support and performance requirements for vibration, deflection and attachment methods. These included 1.2-million-lb silos cantilevered 67 ft above the sixth level and 800,000-lb silos cantilevered approximately 50 ft above the fifth level.

More than 400 penetrations were required through the support deck and diaphragm, ranging from 10 in. in diameter up to 20 sq ft. Provisions for future penetrations also were required, which made reinforcing the existing structure more challenging, he notes. Likewise, shear-wall locations were restricted owing to incoming process piping and connections, as well as the extreme concentrations of massive silos and tanks near the perimeter.

Requirements for seismic control also complicated the design, as the structure is located in Seismic Design Category D. “The nature and magnitude of the seismic-loading demands on the structure required particular attention and careful coordination with everyone on the construction team,” he says.

The framing system featured custom-reinforced, precast concrete shear walls that required special boundary elements. They were used for the lateral load-resisting system on the primary levels and they combined with a steel x-bracing system at the top level. Columns 30 in. square supported heavy T-, L-, and R-beams, which in turn supported a combination of tees and flat slabs. To ensure structural integrity between the diaphragm and the external shear walls, connections between the two were designed with an over-strength factor and the diaphragms were designed to behave elastically.

“The precast system could meet design requirements that typical steel construction could not provide, and the cost parameters eliminated consideration of custom-steel components not readily available,” explains Primm. “Many of the inherent characteristics of precast concrete, such as fire resistance and durability, would have created additional cost with the steel system.”

A dynamic modal analysis performed by the engineering team reconciled the structure’s performance requirements and the magnitude of the forces. “Compared to the results of a static analysis alone, the dynamic analysis prevented over-engineering,” he explains. The base shear and the effect on the shear-wall system were reduced by a factor of nearly 2.25 as a result of the analysis, which significantly lowered connection, reinforcement and labor costs.

“The heavy elevated loads and sensitive equipment of the process facility placed significant demands on the gravity load-resisting system,” he says. “The framework had to be engineered to reduce vibration from operating machinery, as well as control deflection from elevated loads.” Designing the gravity load-resisting system was further compli-cated by vessel- and process-piping penetrations and the need to address the flexibility to handle future possible penetrations.

A 5- to 6-in. contoured topping slab contained three layers of diaphragm reinforcement (north-south, east-west and diagonal) to help transfer the seismic diaphragm forces around the penetrations to the external shear walls. A combination of collector elements and drag struts were used to properly distribute these forces through the diaphragm.

“The precast, prestressed solution for this industrial facility saved the owner money from the outset,” says Primm. “It put the facility into operation a minimum of six months faster than the original all-steel option could have done. The precast concrete system offered a package of benefits and advantages that no other construction type could have offered.”
This strong looking total-precast, prestressed concrete building, which was originally intended to be a steel-framed structure, makes good use of double tees with blockouts to achieve its goals. This project provides a good, muscular expression of the structure while offering good constructability. More than that, it offers lower initial cost and long-term flexibility and durability.